Functional characterization of the gene (NPHP4) causing nephronophthisis type 4. Nephronophthisis (NPHP), an autosomal-recessive cystic kidney disease, constitutes the most frequent genetic cause of chronic renal failure in the first two decades of life. Histologically, the disease is characterized by disrupted tubular basement membrane structure, renal tubular cell atrophy, interstitial fibrosis and cyst formation. In a subset of patients with NPHP there is an association with retinitis pigrnentosa, known as Senior-Loken syndrome (SLS). We have previously identified by positional cloning the gene (NPHP1) for juvenile nephronophthisis. Its gene product "nephrocystin" interacts with signaling proteins that regulate actin organization in the cytoskeleton. We have also identified by positional cloning the gene (NPHP3), mutations in which cause NPHP type 3 and the mouse renal cystic phenotype pcy. In addition, using a candidate approach, we have identified mutations in the human inversin gene as causing NPHP type 2 and demonstrated its expression in primary cilia of renal tubule cells, thus linking the pathogenesis of NPHP to disease mechanisms of polycystic kidney disease. Recently, we have identified by positional cloning the gene (NPHP4) causing NPHP type 4 and SLS type 4. NPHP4 is unique to human and mouse genomes and encodes a novel protein, nephroretinin, which is conserved in the nematode C. elegans. We generated first functional data by demonstrating that, i) nephroretinin is expressed in primary cilia of renal epithelial cells, ii) nephroretinin localizes to specific cUiated neurons in C. elegans which express other proteins relevant for renal cystic disease, and iii), NPHP4 mutations exhibit oligogenic inheritance with other NPHP genes. This proposal is aimed at the functional characterization of the novel NPHP4 gene product "nephroretinin" that we identified. Specifically, we propose to: 1) Determine how oligogenic mutations in nephronophthisis genes influence genotype/phenotype relationships; 2) Characterize the function of the NPHP4 gene and its role in the pathogenesis of NPHP type 4; 3) Generate and characterize mouse models of targeted disruption of the Nphp4 and Nphpl genes. Since nephroretinin represents a novel gene product, we expect these studies to provide new insights into disease mechanisms of renal interstitial fibrosis and cyst development in developing and adult kidney and into the function of the retina.